Solve NameError: name '[user input]' is not defined in Python2

```sh
$ python2 send.py
Welcome to quietnet. Use ctrl-c to exit
> hello
Traceback (most recent call last):
  File "send.py", line 51, in <module>
    message = input("> ")
  File "<string>", line 1, in <module>
NameError: name 'hello' is not defined
```

This problem is caused by the difference of builtin function `input` between Python3 and Python2.
- In Python2 `input()` is equivalent to `eval(raw_input())`
- In Python3 `raw_input()` is renamed to `input()`

This patch should make `send.py` work on both Python2 and Python3.